Systems and Methods to Balance Solar Panels in a Multi-Panel System

ABSTRACT

Systems and methods to balance currents among a plurality of photovoltaic units connected in series. In aspect, a management unit is coupled between a photovoltaic energy production unit and a string of energy production units. The management unit has an energy storage element (e.g., a capacitor) connected to the photovoltaic energy production unit. The management unit further has a switch to selectively couple to the energy storage element and the photovoltaic energy production unit to the string. The management unit allows the current in the string to be larger than the current in the photovoltaic energy production unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of application Ser. No. 12/411,317,filed Mar. 25, 2009, now U.S. Pat. No. 7,602,080, which claims thebenefit of provisional application Ser. No. 61/200,601, filed Dec. 2,2008 and provisional application Ser. No. 61/200,279, filed Nov. 26,2008, the disclosures of which are hereby incorporated herein byreference.

FIELD OF THE TECHNOLOGY

At least some embodiments of the disclosure relate to photovoltaicsystems in general, and more particularly but not limited to, improvingthe energy production performance of photovoltaic systems.

BACKGROUND

Solar system installers take a large guard band (or safety margin) tomake sure the voltages don't cross the 600V or 1000V limits in theUnited States and the European Union, respectively. That limitationinhibits them from installing more solar panel modules, often referredto as “modules” or “panels,” in series to reduce the cost of combinerboxes or string inverters. When solar modules are connected in series orin mesh configurations, there can be a problem in which weaker modulesnot only produce less energy but also affect other modules' capabilitiesto deliver energy in the same string or wiring section.

SUMMARY OF THE DESCRIPTION

Systems and methods to balance currents among a plurality ofphotovoltaic units connected in series are described herein. Someembodiments are summarized in this section.

In one aspect, an apparatus includes: a photovoltaic energy productionunit to generate electricity; and a management unit coupled between thephotovoltaic energy production unit and a series connection of energyproduction units. The management unit has at least a first switch, viawhich the photovoltaic energy production unit generating a first currentis to provide electricity to the series connection of energy productionunits. The management unit is configured to allow a second current,larger than the first current, to flow through the series connection ofenergy production units.

In one embodiment, the energy production unit is at least one solar cellof a solar panel; and the management unit has no inductor and isintegrated on the solar panel. For example, the energy production unitmay be a subset of photovoltaic cells in a string on a solar panel, orthe entire set of photovoltaic cells of a solar panel.

In one embodiment, the management unit further includes an energystorage unit connected to the photovoltaic energy production unit. Whenthe first switch is turned on, the energy production unit provides thefirst current to the series connection of energy production units, theenergy storage unit provides a third current, and the second current inthe series connection is equal to or larger than a sum of the firstcurrent and the third current. When the first switch is turned off, theenergy production unit and the energy storage unit are electronicallydisconnected from the series connection of energy production units, andthe management unit provides at least one path for the series connectionof energy production units.

In one embodiment, the energy storage unit includes a capacitor coupledin parallel with the energy production unit. When the first switch isturned on, an output voltage of the local management unit issubstantially equal to an output voltage of the energy production unit.

In one embodiment, the at least one path includes at least one of: adiode, a second switch that is turned off when the first switch isturned on, and a synchronous rectifier.

In one embodiment, the management unit further includes a controller tocontrol the first switch according to a duty cycle and/or at least oneof: a phase shift, and a synchronization pulse. The controller may beconfigured to control the first switch based on one of: at least oneoperating parameter (e.g., current, voltage, and temperature) associatedwith the energy production unit, at least one operating parameter (e.g.,current, voltage, and temperature) of a separate energy production unit,and a control signal received from a remote unit (e.g., duty cycle,phase, voltage, power).

In one embodiment, the duty cycle is determined based on a maximum powerpoint of the energy production unit, based on a maximum current of theenergy production unit, based on a voltage ratio relative to thestrongest unit on the string, based on a power ratio relative to thestrongest unit on the string, based on a maximum power point voltageratio relative to the strongest unit on the string, and/or based on amaximum power point power ratio relative to the strongest unit on thestring.

In one embodiment, the management unit is a first management unit andthe photovoltaic energy production unit is a first photovoltaic energyproduction unit; and the apparatus further includes: a second managementunit and a second photovoltaic energy production unit. The secondmanagement unit is connected to the first management unit in series. Thesecond management unit has at least a second switch. The secondphotovoltaic energy production unit is to provide electricity to theseries connection of energy production units via the second switch ofthe second management unit. The second management unit is to allow thesecond current, larger than a current from the second photovoltaicenergy production unit, to flow through the series connection of energyproduction units.

In another aspect, a method includes: providing a management unit havinga first switch to couple a solar energy production unit to a seriesconnection of energy production units; and determining at least oneparameter to control the first switch. The management unit has an energystorage unit coupled to the solar energy production unit. When the firstswitch is turned on, the solar energy production unit provides a firstcurrent to the series connection of energy production units, the energystorage unit provides a second current, and a third current in theseries connection of energy production units is equal to or larger thana sum of the first current and the second current. When the first switchis turned off, the solar energy production unit and the energy storageunit are disconnected from the series connection of energy productionunits, and the management unit provides at least one path for the seriesconnection of energy production units.

In one embodiment, the determining of the parameter includes computing aduty cycle to control the first switch based on at least one operatingparameter of the solar energy production unit.

In one embodiment, the at least one operating parameter includes anoperating voltage of the solar energy production unit; and the methodfurther includes: receiving operating voltages of a plurality of solarenergy production units that are connected in series via a plurality ofmanagement units respectively; and identifying a first voltage among theoperating voltages. The duty cycle is then computed according to afunction of the first voltage and the operating voltage of the solarenergy production unit.

In one embodiment, the solar energy production unit is a first solarenergy production unit of the plurality of solar energy productionunits; and when a second solar energy production unit of the pluralityof solar energy production units provides a highest power among theplurality of solar energy production units, the first voltage is anoperating voltage of the second solar energy production unit, and theduty cycle is based at least in part on a ratio between the operatingvoltages of the first and second solar energy production units.

In one embodiment, the method further includes: adjusting the duty cycleuntil a decrease in the operating voltage of the second solar energyproduction unit is detected; and undoing an adjustment to the duty cyclethat causes the decrease.

In another embodiment, the method further includes: adjusting the dutycycle until a decrease in the operating voltage of the second solarenergy production unit is detected; and in response to the decrease,decreasing a duty cycle for a local management unit coupled to thesecond solar energy production unit to increase the operating voltage ofthe second solar energy production unit. In one embodiment, the dutycycle for the second solar energy production unit is decreased until theoperating voltage of the second solar energy production unit ismaximized.

In one embodiment, the method further includes: identifying a voltage ofthe solar energy production unit at a maximum power point based on theat least one operating parameter; and adjusting the duty cycle to changean operating voltage of the solar energy production unit towards thevoltage at the maximum power point.

In one embodiment, the solar energy production unit is a first solarenergy production unit of the plurality of solar energy productionunits; and the method further includes: receiving operating parametersof a plurality of solar energy production units connected in series;identifying a first maximum power point voltage of the first solarenergy production unit based on the at least one operating parameter;identifying a second solar energy production unit having an operatingvoltage highest among the plurality of solar energy production units;identifying a second maximum power point voltage of the second solarenergy production unit; computing a target voltage based on the firstmaximum power point voltage and the second maximum power point voltage;and adjusting the duty cycle to drive an operating voltage of the firstsolar energy production unit to the target voltage.

In one embodiment, the method further includes adjusting the duty cycleto increase the first current.

In one embodiment, the method further includes adjusting the duty cycleto change an operating voltage of the solar energy production unit toincrease an output power of an entire string of solar energy productionunits.

The disclosure includes methods and apparatuses which perform thesemethods, including data processing systems which perform these methods,and computer readable media containing instructions which when executedon data processing systems cause the systems to perform these methods.

Other features will be apparent from the accompanying drawings and fromthe detailed description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not limitation inthe figures of the accompanying drawings in which like referencesindicate similar elements.

FIGS. 1-3 illustrate local management units according to someembodiments.

FIG. 4 illustrates a photovoltaic system according to one embodiment.

FIG. 5 illustrates a solar panel according to one embodiment.

FIGS. 6-8 show methods to improve performance of a photovoltaic systemaccording to some embodiments.

DETAILED DESCRIPTION

The following description and drawings are illustrative and are not tobe construed as limiting. Numerous specific details are described toprovide a thorough understanding. However, in certain instances, wellknown or conventional details are not described in order to avoidobscuring the description. References to one or an embodiment in thepresent disclosure are not necessarily references to the sameembodiment; and, such references mean at least one.

When solar modules are connected in series or mesh configuration, therecan be a problem in which weaker modules not only produce less energybut also affect other modules in the same string or wiring section. Bymeasuring one can determine that a few modules are weaker than theothers in most commercially installed strings. Thus, the string isgenerating less power than the sum available at each module if moduleswere operated separately.

At least one embodiment of the present disclosure provides methods andsystems to switch on and off weak modules in the string in a way thatthe current on the string bus from the good modules won't be affected bythe weak modules.

FIGS. 1-3 illustrate local management units according to someembodiments. In FIGS. 1-3, local management units (101) are used toswitch on and off the solar module (102) periodically to improve theenergy production performance of the photovoltaic systems connected, atleast in part, in series.

In FIG. 1, a management unit (101) is local to the solar module (102)and can be used to periodically couple the solar module (102) to theserial power bus (103) via the switch Q1 (106), to improve the totalpower output for the string of solar modules connected to the serialpower bus in series.

The local management unit (LMU) (101) may include a solar modulecontroller to control the operation of the solar module (102) and/or alink module unit to provide connectivity to the serial power bus (103)for energy delivery and/or for data communications.

In one embodiment, the command to control the operation of the switch Q1(106) is sent to the local management unit (101) over the photovoltaic(PV) string bus (power line) (103). Alternatively, separate networkconnections can be used to transmit the data and/or commands to/from thelocal management unit (101).

In FIGS. 1 and 2, the inputs (104 a, 104 b, 104 c) to the localmanagement unit (101) are illustrated separately. However, the inputs(104 a, 104 b, 104 c) are not necessarily communicated to localmanagement unit (101) via separate connections. In one embodiment, theinputs are received in the local management unit via the serial powerbus (103).

In FIG. 1, the solar module (102) is connected in parallel to thecapacitor C1 (105) of the local management unit (101). The diode D1(107) of the local management unit (101) is connected in series in theserial power bus (103) which may or may not be part of an overall meshconfiguration of solar modules. The switch Q1 (106) of the localmanagement unit can selectively connect or disconnect the solar module(102) and the capacitor C1 (105) from a parallel connection with thediode D1 (107) and thus connect or disconnect the solar module (102)from the serial power bus (103).

In FIG. 1, a controller (109) of the local management unit (101)controls the operation of the switch (106) according to the parameters,such as duty cycle (104 a), phase (104 b) and synchronization pulse (104c).

In one embodiment, the controller (109) receives the parameters (104 a,104 b, 104 c) from a remote management unit via the serial power bus(103) or a separate data communication connection (e.g., a separate databus or a wireless connection). In some embodiment, the controller (109)may communicate with other local management units connected on theserial power bus (103) to obtain operating parameters of the solarmodules attached to the serial power bus (103) and thus compute theparameters (e.g., 104 a and 104 b) based on the received operatingparameters. In some embodiment, the controller (109) may determine theparameter (e.g., 104 a and 104 b) based on the operating parameters ofthe solar module (102) and/or measurements obtained by the controller(109), without communicating with other local management units of othersolar modules, or a remote system management unit.

In FIG. 2, a system (100) has a local management unit (101) coupled tothe solar module (102). The local management unit (101) is connectedbetween the solar module (102) and the string bus (103) to improve thetotal power output for the whole string on the serial power bus (103).Commands to the local management unit (101) can be sent over thephotovoltaic (PV) string bus (power line) (103). To make the figure moreclear, the inputs (104 a, 104 b, 104 c) to the controller (109) of thelocal management unit (101) were drawn separately, which does notnecessarily indicate that the inputs (104 a, 104 b, 104 c) are providedvia separate connections and/or from outside the local management unit(101). For example, in some embodiments, the controller (109) maycompute the parameters (104 a, 104 b, 104 c) based on measurementsobtained at the local management unit (101), with or without datacommunications over the serial power bus (103) (or a separate datacommunication connection with other management units).

In FIG. 2, the local management unit (101) is connected in one side tothe solar module (102) in parallel and on the other side in series to astring of other modules, which may or may not be part of an overall meshconfiguration. The local management unit (101) may receive, amongothers, three inputs or types of input data, including a) requested dutycycle (104 a), which can be expressed as a percentage (e.g., from 0 to100%) of time the solar module (102) is to be connected to the serialpower bus (103) via the switch Q1 (106), b) a phase shift (104 b) indegrees (e.g., from 0 degree to 180 degree) and c) a timing orsynchronization pulse (104 c). These inputs (e.g., 104 a, 104 b and 104c) can be supplied as discrete signals, or can be supplied as data on anetwork, or composite signals sent through the power lines orwirelessly, and in yet other cases, as a combination of any of theseinput types.

In FIG. 2, the local management unit (101) periodically connects anddisconnects the solar module (102) to and from the string that forms theserial power bus (103). The duty cycle (104 a) and the phase (104 b) ofthe operation of the switch Q1 (106) can be computed in a number of waysto improve the performance of the system, which will be discussedfurther below.

In FIG. 2, the local management unit (101) includes a capacitor C1 (105)and a switch Q1 (106), as well as a diode D1 (107). In FIG. 2, the diodeD1 (107) is supplemented with an additional switch Q2 (108), which actsas a synchronous rectifier to increase efficiency. In one embodiment,the additional switch Q2 (108) is open (turned off) when the switch Q1(106) is closed (turned on) to attach the solar module (102) (and thecapacitor C1 (105)) to the serial power bus (103).

In some cases, a filter (not shown), including a serial coil and aparallel capacitor, is also used. The filter may be placed at the localmanagement unit or placed just before the fuse box or inverter, or bepart of either one of those.

In FIG. 2, the controller (109) is used to process the input signals(e.g., 104 a, 104 b, 104 c) and drive the switches Q1 (106) and Q2(108). In one embodiment, the controller (109) is a small single chipmicro controller (SCMC). For example, the controller (109) may beimplemented using Application-Specific Integrated Circuit (ASIC) orField-Programmable Gate Array (FPGA). The controller (109) can even beimplemented in discrete, functionally equivalent circuitry, or in othercases a combination of SCMC and discrete circuitry.

In one embodiment, the controller (109) is coupled to the solar module(102) in parallel to obtain power for processing; and the controller(109) is coupled to the serial power bus (103) to obtain signalstransmitted from other management units coupled to the serial power bus(103).

By switching the module (102) (or groups of cells, or a cell) on and offto the string periodically, the local management unit (101) may lowerthe voltage reflected to the string bus (103) (e.g., a lower averagevoltage contributed to the string bus) and can cause the currentreflected to the string bus (103) to be higher, nearer the level itwould be if the module was not weak, generating a higher total poweroutput.

In one embodiment, it is preferable to use different phases to operatethe switches in different local management units on a string to minimizevoltage variance on the string.

In FIG. 3, the local management unit (101) provides two connectors (112and 114) for serial connections with other local management unit (101)to form a serial power bus (103). The controller (109) controls thestates of the switches Q1 (106) and Q2 (108).

In FIG. 3, when the controller (109) turns on the switch (106), thepanel voltage and the capacitor C1 (105) are connected in parallel tothe connectors (112 and 114). The output voltage between the connectors(112 and 114) is substantially the same as the output panel voltage.

In FIG. 3, during the period the switch (106) is turned off (open), thecontroller (109) turns on (closes) the switch (108) to provide a patharound the diode D1 (107) to improve efficiency.

In FIG. 3, when the switch (106) is turned off (open), the panel voltagecharges the capacitor C1 (105), such that when the switch (106) isturned on, both the solar panel and the capacitor (105) providescurrents going through the connectors (112 and 114), allowing a currentlarger than the current of the solar panel to flow in the string (theserial power bus (103)). When the switch (106) is turned off (open), thediode D1 (107) also provides a path between the connectors (112 and 114)to sustain the current in the string, even if the switch (108) is offfor some reasons.

In one embodiment, the controller (109) is connected (not shown in FIG.3) to the panel voltage to obtain the power for controlling the switchesQ1 (106) and Q2 (108). In one embodiment, the controller (109) isfurther connected (not shown in FIG. 3) to at least one of theconnectors to transmit and/or receive information from the string. Inone embodiment, the controller (109) includes sensors (not shown in FIG.3) to measure operating parameters of the solar panel, such as panelvoltage, panel current, temperature, light intensity, etc.

FIG. 4 illustrates a photovoltaic system (200) according to oneembodiment. In FIG. 4, the photovoltaic system 200 is built from a fewcomponents, including photovoltaic modules (201 a, 201 b, . . . , 201n), local management unit units (202 a, 202 b, . . . , 202 n), aninverter (203), and a system management unit (204).

In one embodiment, the system management unit (204) is part of theinverter (203), the combiner box (206), a local management unit, or astand-alone unit. The solar modules (201 a, 201 b, . . . , 201 n) areconnected in parallel to the local management unit units (202 a, 202 b,. . . , 202 n) respectively, which are connected in series to form astring bus (205), which eventually is connected to an inverter (203) andthe management unit (204).

In FIG. 4, the string bus (205) can be connected to the inverter (203)directly or as part of a mesh network or combiner boxes or fuse boxes(not shown). An isolated local management unit can be used as a combinerbox (206) to adjust all voltages before connecting to the inverter(206); or, a single or multi-string inverter can be used. To limit thechanges in the voltage of the bus, the management unit (204) may assigna different phase for each of the local management units (202 a, 202 b,. . . , 202 n). In one embodiment, at any given time, a maximum of apredetermined number of solar modules (e.g., one single solar module)are disconnected from the string bus (205).

In one embodiment, beyond the module connection the local managementunits can have the signal inputs, including but not limited to dutycycle (104 a), phase (104 b) and synchronization pulse (104 c) (e.g., tokeep the local management units synchronized). In one embodiment, thephase (104 b) and the synchronization pulse (104 c) are used to furtherimprove performance, but the local management unit (101) can workwithout them.

In one embodiment, the local management unit may provide output signals.For example, the local management unit (101) may measure current andvoltage at the module side and optionally measure current and voltage inthe string side. The local management unit (101) may provide othersuitable signals, including but not limited to measurements of light,temperature (both ambient and module), etc.

In one embodiment, the output signals from the local management unit(101) are transmitted over the power line (e.g., via power linecommunication (PLC)), or transmitted wirelessly.

In one embodiment, the system management unit (204) receives sensorinputs from light sensor(s), temperature sensor(s), one or more each forambient, solar module or both, to control the photovoltaic system (200).In one embodiment, the signals may also include synchronization signals.For example, a management unit can send synchronization signalsperiodically to set the timing values, etc.

Using the described methods the local management unit can be a verynon-expensive and reliable device that can easily increase thethroughput of a photovoltaic solar system by a few (e.g., signal or lowdouble digits) percentage points. These varied controls also allowinstallers using this kind of system to control the VOC (open circuitvoltage) by, for example by shutting off some or all modules. Forexample, by using the local management units of the system, a fewmodules can be disconnected from a string if a string is getting to theregulatory voltage limit, thus more modules can be installed in astring.

In some embodiments, local management units can also be used within thesolar panel to control the connection of solar cells attached to stringsof cells within the solar panel.

FIG. 5 illustrates a solar panel according to one embodiment. In oneembodiment, the solar panel (300) has a few strings of solar cells(e.g., three solar cell strings per module). In FIG. 5, a localmanagement unit (101) can be applied to a group of cells (301) within astring of an individual solar panel (300), or in some cases to each cell(301) in a solar panel (300).

In FIG. 5, a group of solar cells (301) that are attached to a localmanagement unit (101) may be connected to each other in series, inparallel, or in a mesh configure. A number of local management units(101) connect the groups of the solar cells (301) in a string to provideoutput for the solar panel (300).

Some embodiments of the disclosure includes methods to determine theduty cycles and/or phases for local management units connected to astring or mesh of solar modules.

In some embodiments, the duty cycle of all local management units in astring or mesh can be changed, to increase or decrease the stringvoltage. The duty cycles may be adjusted to avoid exceeding the maximumvoltage allowed. For example, the maximum voltage may be limited by thecombiner box (206), the inverter (203), or any other load connected tothe string bus (205), or limited by any regulations applicable to thatsystem. In some embodiments, the duty cycles are adjusted to align thevoltage of multiple strings.

In some embodiments, the duty cycle of one local management unit (101)in a string can be changed to cause higher current in that localmanagement unit (101) and overall higher power harvesting.

In one embodiment, the duty cycles are computed for the solar modulesthat are connected to a string via the corresponding local managementunits. The duty cycles can be calculated based on the measured currentand voltages of the solar modules and/or the temperatures.

After an initial set of duty cycles is applied to the solar modules, theduty cycles can be further fine tuned and/or re-adjusted to changes,such as shifting shading etc., one step a time, to improve powerperformance (e.g., to increase power output, to increase voltage, toincrease current, etc.). In one embodiment, target voltages are computedfor the solar modules, and the duty cycles are adjusted to drive themodule voltage towards the target voltages.

The methods to compute the duty cycles of the solar modules can also beused to compute the duty cycles of the groups of solar cells within asolar module.

FIGS. 6-8 show methods to improve performance of a photovoltaic systemaccording to some embodiments.

In FIG. 6, at least one operating parameter of a solar energy productionunit coupled to a string via a management unit is received (401) andused to identify (403) a duty cycle for the management unit to connectthe solar energy production unit to string. The solar energy productionunit may be a solar module, a group of solar cells within a solarmodule, or a single solar cell in a string in a solar module. The dutycycle is adjusted (405) to optimize the performance of the solar energyproduction unit and/or the string.

For example, the duty cycle can be adjusted to increase the current inthe string and/or the solar energy production unit, to increase theoutput power of the string and/or the solar energy production unit, toincrease the voltage of the solar energy production unit, etc.

In FIG. 7, the operating voltages of a plurality of solar panelsconnected in series are received (421) and used to identify (423) asecond solar panel having the highest operating voltage (highest outputpower) in the string.

In FIG. 7, a duty cycle of a first solar panel is computed (425) basedon a ratio in operating voltage between the first and second solarpanels. Alternatively, the duty cycle can be computed based on a ratioin output power between the first and second solar panels.Alternatively, the duty cycle can be computed based on a ratio betweenthe first and second solar panels in estimated/computed maximum powerpoint voltage. Alternatively, the duty cycle can be computed based on aratio between the first and second solar panels in estimated/computedmaximum power point power.

The duty cycle of the first solar panel is adjusted (427) to improve theperformance of the first solar energy production unit and/or the string,until a decrease in the operating voltage of the second solar panel isdetected. For example, the duty cycle of the first solar panel can beadjusted to increase the total output power of the string, to increasethe current of the string, to increase the current of the first solarpanel, to drive the voltage of the first solar panel towards a targetvoltage, such as its maximum power point voltage estimated based on itscurrent operating parameters, such as temperature or a voltagecalculated using its estimated maximum power point voltage.

In FIG. 7, in response to the detected decrease in the operating voltageof the second solar panel which had the highest operating voltage, theadjustment in the duty cycle of the first solar panel that causes thedecrease is undone/reversed (429).

In FIG. 7, the duty cycle of the second solar panel is optionallydecreased (431) to increase the operating voltage of the second solarpanel. In some embodiments, the strongest solar panel (or strong panelswithin a threshold from the strongest panel) is not switched off line(e.g., to have a predetermined duty cycle of 100%).

In one embodiment, the duty cycle of the second solar panel isrepeatedly decreased (429) until it is determined (431) that thedecrease (429) in the duty cycle of the second solar panel cannotincrease the voltage of the second solar panel.

In FIG. 8, operating parameters of a plurality of solar panels connectedin a string are received (441) and used to identify (443) a firstmaximum power point voltage of a first solar panel. A second solar panelhaving the highest operating voltage (or output power) in the string isidentified. A second maximum power point voltage of the second solarpanel is identified (447) based on the received operating parameters andused to compute (449) a target voltage for the first solar energyproduction unit. In one embodiment, the target voltage is a function ofthe first and second maximum power point voltages and the highestoperating voltage identified (445) in the second solar panel in thestring. The duty cycle of the first solar energy production unit isadjusted to drive the operating voltage of the first solar panel towardsthe target voltage.

Alternatively, the target voltage may be the set as the first maximumpower point voltage of the first solar panel.

In one embodiment, to adjust voltage a same factor is applied to allmodules in that string. For example, in a case of a first module A1 thatis producing only 80%, and the voltage of the whole string needs to be5% lower, the duty cycle of A1 is 80% multiplied the duty cycle appliedto the whole string (which is Y in this example) so module A1 then hasY×0.8 as duty cycle.

In some embodiments, the system management unit (204) and/or the localmanagement units (e.g., 202 a, 202 b, . . . , 202 n) are used solely orin combination to determine the parameters to control the operations ofthe switches.

For example, in one embodiment, a system management unit (204) is the“brain” of the system, which decides on the duty cycle and phaseparameters.

For example, in another embodiment, each local management unitbroadcasts information to the other local management units on the stringto allow the individual local management units to decide their own dutycycle and phase parameters.

In some embodiment, a local management unit may instruct one or moreother local management units to adjust duty cycle and phase parameters.For example, the local management units on a string bus (205) may electone local management unit to compute the duty cycle and phase parametersfor other local management units on the string.

For example, in some embodiment, the system management unit (204) maydetermine one or more global parameters (e.g., a global duty cycle, themaximum power on the string, the maximum voltage on the string, etc.),based on which individual local management units adjust their own dutycycles.

In some embodiments, a local management unit may determine its own dutycycles without relying upon communicating with other management units.For example, the local management unit may adjust its duty cycle forconnecting its solar module to the string to operate the solar module atthe maximum power point.

In one embodiment, module voltage are measured by the local managementunits in the same string at substantially/approximately the same timeand used to identify the strongest solar module. A strongest solarmodule provides the most power in the string. Since the modules areconnected in series, the solar module having the highest module voltagein the string can be identified as the strongest solar module. In someembodiment, the operating voltage and current of the solar module aremeasured to determine the power of the solar module.

In one embodiment, after the highest module voltage V_(m) in the stringis identified, the duty cycle for each module can be computed as afunction of a ratio between the module voltage V of the module and thehighest module voltage V_(m). For example, the duty cycle for a modulecan be computed as 1−((V_(m)−V)/V_(m))=V/V_(m).

In one embodiment, the system management (204) may identify the highestmodule voltage from the module voltages received from the localmanagement units (202 a, 202 b, . . . , 202 n), and compute the dutycycles for the corresponding local management units (202 a, 202 b, . . ., 202 n).

In one embodiment, the local management units (202 a, 202 b, . . . , 202n) may report their module voltages on the string bus (205) to allowindividual local management units (202 a, 202 b, . . . , 202 n) toidentify the highest module voltage and compute the duty cycles, withoutrelying upon the system management unit (204).

In one embodiment, one of the local management units (202 a, 202 b, . .. , 202 n) may identify the highest module voltage and/or compute theduty cycles for the other local management units (202 a, 202 b, . . . ,202 n).

In one embodiment, the duty cycles are determined and/or adjustedperiodically.

In one embodiment, after the duty cycles for the solar modules on thestring are set based on the module voltage ratio relative to the highestmodule voltage in the string, the duty cycles can be fine tuned toincrease the power performance. The duty cycles can be fine tuned onestep a time, until a decrease of voltage of the module with the highestpower is detected. In response to the detected decrease, the last changethat caused the decrease can be reversed (undone). The fine tuning ofthe duty cycles can be used to reach the peak performance point (e.g.,for maximum power point tracking).

In one embodiment, after the strongest module is identified, the dutycycles of the solar modules on the string are adjusted until the modulewith the highest power in the string decrease its voltage. Sincedecreasing the duty cycle of a solar module decreases the time periodthe module is connected to the string and thus increases its voltage,the duty cycle of the module with the highest power in the string can bedecreased to increase its voltage, in response to the decrease in itsvoltage caused by the adjustment to the duty cycles of other solarmodules on the string. For example, the duty cycle of the module withthe highest power in the string can be decreased until its voltage ismaximized.

In one embodiment, the local management unit measures module and ambienttemperatures for some methods to determine the duty cycles. For example,the operating parameters measured at the local management units (e.g.,202 a, 202 b, . . . , 202 n), such as module temperature, can be usedcompute the estimated voltages of the solar modules at their maximumpower points. For example, a formula presented by Nalin K. Gautam and N.D. Kaushika in “An efficient algorithm to simulate the electricalperformance of solar photovoltaic arrays”, Energy, Volume 27, Issue 4,April 2002, pages 347-261, can be used to compute the voltage V_(mp) ofa solar module at the maximum power point. Other formulae can also beused. Once the maximum power point voltage V_(mp) of a solar module iscomputed or estimated, the duty cycle of the solar module connected to astring can be adjusted to drive the module voltage to thecomputed/estimated maximum power point voltage V_(mp), since decreasingthe duty cycle of a solar module normally increases its voltage.

In one embodiment, a local management unit may adjust the duty cycle ofthe solar module connected to the local management unit to change themodule voltage to the computed/estimated maximum power point voltageV_(mp), without having to communicate with other management units.

In one embodiment, a local management unit (or a system management unit)may adjust the duty cycle of the solar module connected to the localmanagement unit to perform maximum power point tracking.

In one embodiment, after identifying the strongest module andcomputing/estimating the maximum power point voltage V_(mpm) of thestrongest module, the duty cycle for each module on a string can becomputed as a function of a ratio between the maximum power pointvoltage V_(mp) of the module and the maximum power point voltage V_(mpm)of the strongest module. For example, the duty cycle for a module can becomputed as 1−((V_(mpm)−V_(mp))/V_(mpm))=V_(mp)/V_(mpm). The duty cyclecan be periodically updated, based on the current operating parametersmeasured, and/or fine tuned until a decrease in the voltage of thestrongest module is detected.

Alternatively, a target voltage for each module on the string can becomputed as a function of a ratio between the maximum power pointvoltage V_(mp) of the module and the maximum power point voltage V_(mpm)of the strongest module. For example, the target voltage for a modulecan be computed as V_(m)×V_(mp)/V_(mpm), where V_(m) is the measuredvoltage of the strongest module. The duty cycle of the module can bechanged to drive the module voltage of the module towards the targetvoltage.

In one embodiment, after identifying the strongest module andcomputing/estimating the maximum power point power P_(mpm) of thestrongest module, the duty cycle for each module on a string can becomputed as a function of a ratio between the maximum power point powerP_(mp) of the module and the maximum power point power P_(mpm) of thestrongest module. For example, the duty cycle for a module can becomputed as 1−((P_(mpm)−P_(mp))/P_(mpm))=P_(mp)/P_(mpm). The duty cyclecan be periodically updated, based on the current operating parametersmeasured, and/or fine tuned until a decrease in the voltage of thestrongest module is detected, since decreasing the duty cycle normallyincreases the module voltage.

In one embodiment, a target voltage for each module on the string can becomputed as a function of a ratio between the maximum power point powerP_(mp) of the module and the maximum power point power P_(mpm) of thestrongest module. For example, the target voltage for a module can becomputed as V_(m)×P_(mp)/P_(mpm), where V_(m) is the measured voltage ofthe strongest module. The duty cycle of the module can be changed todrive the module voltage of the module towards the target voltage, sincedecreasing the duty cycle normally increases the module voltage.

In one embodiment, the duty cycle for each local management unit ischanged to increase the current of the solar module attached to thelocal management unit (e.g., based on the measurement of the voltage andcurrent of the solar module), until the maximum current is achieved.This method assumes that string maximum power can be achieved with someaccuracy by driving each local management unit to maximum current. Inone embodiment, the voltages and currents of the solar modules aremeasured for tuning the duty cycles for maximum power point tracking forthe string. The measurements of the voltages and currents of the solarmodules also enable the local management units to additionally serve asa module level monitoring system.

The duty cycles can be adjusted by the system management unit (e.g.,204) based on the measurements reported by the local management units(e.g., 202 a, 202 b, . . . , 202 n), or adjusted directly by thecorresponding local management units (e.g., 202 a, 202 b, . . . , 202n).

In one embodiment, during the process of setting and/or tuning the dutycycles, the maximum power point tracking operation by the inverter (203)is frozen (temporarily stopped). Light intensity at the solar modules ismonitored for changes. When the light intensity at the solar modulesstabilizes, the voltage and current of the solar modules are measuredfor the determination of the duty cycles. Then normal operation resumes(e.g., unfreezing of maximum power point tracking operation).

In one embodiment, the local management units measure the voltages andcurrents of the solar modules to determine the power of the solarmodules. After identifying the highest power P_(m) of the solar moduleon the string, the duty cycles of the solar modules on the string aredetermined by the power radio relative to the highest power P_(m). Forexample, if a module produces 20 percent less power, it will bedisconnected from the string bus about 20 percent of the time. Forexample, if a module produces power P, its duty cycle can be set to1−((P_(m)−P)/P_(m))=P/P_(m).

In one embodiment, a predetermined threshold is used to select the weakmodules to apply duty cycles. For example, in one embodiment, when amodule produces power less than a predetermine percent of highest powerP_(m), a duty cycle is calculated and applied to the solar module. Ifthe module is above the threshold, the module is not disconnected (andthus having a duty cycle of 100%). The threshold may be based on thepower, or based on the module voltage.

In one embodiment, the system management unit (204) finds the dutycycles for the local management units (202 a, 202 b, . . . , 202 n) andtransmits data and/or signals representing the duty cycles to the localmanagement units (202 a, 202 b, . . . , 202 n) via wires or wirelessconnections. Alternatively, the local management units (202 a, 202 b, .. . , 202 n) may communicate with each other to obtain the parameters tocalculate the duty cycles.

In one embodiment, the system management unit (204) knows all thedifferent duty cycles indicated for the local management units (202 a,202 b, . . . , 202 n).

In one embodiment, during power fine tuning, the system management unit(204) sends the appropriate data/signal to the appropriate localmanagement units (202 a, 202 b, . . . , 202 n), and then the systemmanagement unit (204) calculates the total power of the string andcorrects the duty cycle to produce maximum power. Once maximum power isachieved, the duty cycles for the local management units (202 a, 202 b,. . . , 202 n) may be saved in a database and serve as a starting pointfor the corresponding local management units (202 a, 202 b, . . . , 202n) at the same time of day on the next day. Alternatively, a localmanagement may store the duty cycle in its memory for the next day.

The stored duty cycles can be used when there is a fixed shade on themodules, such as a chimney, a tree, etc., which will be the same shadeon any day at the same time. Alternatively, historical data may not besaved, but may be recalculated from scratch on each run, for exampleevery 30 minutes.

In one embodiment, the light intensity at the solar modules is monitoredfor changes. The duty cycles are calculated when the light intensitydoes not change significantly. If there are changes in sun lightradiation at the solar modules, the system will wait until theenvironment stabilizes before applying or adjusting the duty cycles.

In one embodiment, the system management unit (204) can communicate withthe inverter as well. When the environment is not stable (e.g., when thesun light radiation is changing), the inverter may stop maximum powerpoint tracking. In such a situation, the inverter can be set up for itsload, instead of tracking for maximum power point. Instead of using theinverter to perform maximum power point tracking, the system managementunit (204) and the local management units (202 a, 202 b, . . . , 202 n)are used to set the operating parameters and balance the string.

Alternatively, when the environment is not stable but measurements andcalculation are done faster than the MPPT is working, there may be noneed to stop the MPPT on the inverter. Alternatively, when theenvironment is not stable, measurements can be taken few times for thesame radiation until a stable result is achieved.

Many variations may be applied to the systems and methods, withoutdeparting from the spirit of the invention. For example, additionalcomponents may be added, or components may be replaced. For example,rather than using a capacitor as primary energy store, an inductor maybe used, or a combination of inductor and capacitor. Also, the balancebetween hardware and firmware in the micro controllers or processors canbe changed, without departing from the spirit of the invention. In somecases, only some problematic modules may have a local management unit,for example in a shaded or partially shaded or otherwise differentsituation. In yet other cases, local management units of strong modulesmay be virtually shut off. The methods for determining the duty cyclesfor the solar modules can also be used to determine the duty cycles ofgroups of cells connected via local management units in a string withina solar panel/module.

In the foregoing specification, the disclosure has been described withreference to specific exemplary embodiments thereof. It will be evidentthat various modifications may be made thereto without departing fromthe broader spirit and scope as set forth in the following claims. Thespecification and drawings are, accordingly, to be regarded in anillustrative sense rather than a restrictive sense.

1-20. (canceled)
 21. A management unit in a photovoltaic energyproduction system, comprising: a circuit to periodically connect anddisconnect a solar module to and from a power bus according to a dutycycle, the duty cycle being adjustable to operate the solar module atmaximum power point; and a controller to receive operating parametersfrom first management units coupled to the power bus, each of the firstmanagement units to determine operating parameters of a respective solarmodule and transmit the operating parameters of the respective solarmodule to the controller of the management unit, the controller todetermine duty cycles for the first management units based at least inpart on the operating parameters received from the first managementunits.
 22. The management unit of claim 21, wherein the power bus is aserial power bus.
 23. The management unit of claim 22, wherein each ofthe first management units has a switch to periodically connect anddisconnect a respective solar module to and from the power bus; and thecontroller is configured to determine different phases for respectiveswitches in the first management units.
 24. A photovoltaic energyproduction system, comprising: a power bus; a plurality of photovoltaicenergy production units; a plurality of first management units coupledbetween the plurality of photovoltaic energy production units and thepower bus to supply electricity generated by the photovoltaic energyproduction units onto the bus, the first management units to determineoperating parameters of the photovoltaic energy production units, eachof the first management units being capable of adjusting a respectivephotovoltaic energy production unit to a maximum power point of therespective photovoltaic energy production unit; and a second managementsunit in communication with the first management units to receive theoperating parameters of the photovoltaic energy production units, thesecond management unit to determine at least one parameter for the firstmanagement units, the first management units to operate at least one ofthe photovoltaic energy production units at a maximum power pointaccording to the at least one parameter.
 25. The system of claim 24,wherein the power bus is a string bus; and the plurality of firstmanagement units connect the plurality of photovoltaic energy productionunits in series on the string bus.
 26. The system of claim 24, whereinthe second management unit communicates with first management units overthe power bus.
 27. A method, comprising: receiving, from a plurality offirst management units and by a second management unit, operatingparameters of a plurality of solar modules, each of the plurality offirst management units being coupled with a respective solar module ofthe plurality of solar modules to determine operating parameters of therespective solar module and coupled between the respective solar moduleand a power bus to supply electricity generated by the respective solarmodule onto the power bus; determining, by the second management unit,at least one control parameter based on the operating parameters of theplurality of solar modules; and transmitting, by the second managementunit, the at least one control parameter to the first management unitsto operate at least one of the solar modules at maximum power point. 28.The method of claim 27, wherein the power bus comprises a string of theplurality of first management units connected in series.
 29. The methodof claim 27, further comprising: calculating, by the second managementunit, estimated maximum power points of the solar modules, wherein theat least one control parameter is determined based on the estimatedmaximum power points of the solar modules.
 30. The method of claim 27,further comprising: electing the second management unit from a pluralityof management units connected on the power bus.
 31. A photovoltaicenergy production system, comprising: a power bus; a plurality ofphotovoltaic energy production units; a plurality of first managementunits coupled between the plurality of photovoltaic energy productionunits and the power bus to supply electricity generated by thephotovoltaic energy production units onto the bus, the first managementunits to determine operating parameters of the photovoltaic energyproduction units, each of the first management units being capable ofadjusting a respective photovoltaic energy production unit to a maximumpower point of the respective photovoltaic energy production unit; andan inverter coupled to the power bus to receive the electricitygenerated by the photovoltaic energy production units, the inverterconfigured to track maximum power point while receiving the electricityfrom the power bus.
 32. The system of claim 31, wherein the power bus isa string bus; and the plurality of first management units connect theplurality of photovoltaic energy production units in series on thestring bus.
 33. The system of claim 31, wherein the inverter performsmaximum power point tracking on the power bus, while at least one of thefirst management units adjusts a respective photovoltaic energyproduction unit to a maximum power point.
 34. The system of claim 31,further comprising: a second management unit configured to determine atleast one control parameter for the first management units, wherein eachof the first management units is configured to adjust a respectivephotovoltaic energy production unit based on the control parameter. 35.The system of claim 34, wherein the second management unit is configuredto determine a plurality of values of the control parameter for theplurality of first management units respectively.
 36. A method,comprising: transmitting, from a plurality of first management units,operating parameters of a plurality of solar modules, each of theplurality of first management units being coupled with a respectivesolar module of the plurality of solar modules to determine operatingparameters of the respective solar module and coupled between therespective solar module and a power bus to supply electricity generatedby the respective solar module onto the power bus; receiving, by thefirst management units, at least one control parameter to operate atleast one of the solar modules at maximum power point, wherein the atleast one control parameter is determined based on the operatingparameters of the plurality of solar modules; and performing, by aninverter connected to the power bus, maximum power point tracking, whilethe inverter receives electricity from the power bus.
 37. The method ofclaim 36, further comprising: measuring the operating parameters of theplurality of solar modules by the plurality of first management unitswhile the inverter performing maximum power point tracking.
 38. Themethod of claim 36, further comprising: stopping maximum power pointtracking at the inverter while measuring the operating parameters of theplurality of solar modules by the plurality of first management units;and resuming maximum power point tracking at the inverter aftermeasuring the operating parameters of the plurality of solar modules bythe plurality of first management units.
 39. The method of claim 36,wherein at least one of the first management units adjusts at least oneof the solar modules according to the control parameter, while themaximum power point tracking is performed by the inverter.
 40. Themethod of claim 39, wherein the control parameter is selected from thegroup consisting of: duty cycle, phase shift, and synchronization pulse.